Castlefans, it's a double primetime dose of Nathan Fillion this week!Dr. Horrible's Sing-Along Blog, a three-part musical web series created in 2008 by sci-fi mastermind Joss Whedon, will debut tonight, Oct. 9 on The CW! As most Fillion fans already know, Nathan plays Captain Hammer, the archfoe of super villain Dr. Horrible, portrayed by Nathan's real-life friend Neil Patrick Harris. It's the job of superhero Hammer to stop Dr. Horrible from winning the affections of Penny (played by Felicia Day), and entering the Evil League of Evil.Dr. Horrible's Sing-Along Blog is scheduled to air tonight at 10 p.m. ET on The CW. Will you tune in?